One of the problems facing the nuclear industry today is that of disposing of the large amounts of radioactive waste generated by nuclear reactors. As an example, the Savannah River Plant operated by the Department of Energy has generated nearly 300,000 cubic meters of high level radioactive waste since the plant startup in the early 1950's. It is estimated that this figure will increase at the rate of up to 15,000 cubic meters of waste per year. Consequently, as the Savannah River Plant is but one of the many nuclear plants operating today, there is tremendous interest in devising methods to deal with and reduce these huge quantities of radioactive materials by safe and efficient means.
The radioactivity of the soluble waste materials is due primarily to three radionuclides: cesium-137; strontium-90; and plutonium. Of these three substances, cesium-137 accounts, by itself, for more than 99 percent of the radioactivity of the soluble waste materials. Thus, it is especially desirable to be able to concentrate the cesium-137 in order to decrease the total amount of material which it contaminates.
A process has been developed to separate the above three radionuclides from waste solution. This process, described in Lee et al., U.S. Pat. No. 4,432,893, comprises adding sodium tetraphenylborate (NaTPB) and sodium titanate (NaTi.sub.2 O.sub.5 H) to a waste solution, whereby cesium-137 is precipitated out as insoluble CsTPB, and strontium-90 and plutonium are absorbed onto the NaTi.sub.2 O.sub.5 H by means of an ion exchange process to form their respective titanates which are likewise insoluble. After filtration, the remaining aqueous solution has been decontaminated by a factor of 104 for cesium-137, up to 200 for strontium-90, and up to 500 for plutonium. Following filtration, the concentrated, radioactive solids are combined with a sludge fraction of the waste and are then vitrified into borosilicate glass, which in turn is placed in suitable containers for long-term storage.
In the vitrification process, which occurs in a glass melter operated at about 1100.degree. C., cations are reduced to oxides which are incorporated into the glass matrix. Most of the anions and all of the hydrocarbons are oxidized and removed through the off-gas system. For each mole of C.sub.6 H.sub.5 -oxidized, approximately 7.5 moles of oxygen are required and 9 moles of gas are generated. Removal of the organic compounds prior to the vitrification step would decrease both the amount of oxygen required and the volume of gas generated. This, in turn, would decrease the size and complexity of the off-gas system. It is, therefore, highly desirable to develop a means by which the amount of organic compounds in the concentrated radioactive wastes can be decreased in order to facilitate operation of the glass melter.
It has been found that hydrolysis of the organic compounds in acidic solutions results in evolution of organic vapors which are relatively free from radioactivity. In particular, solutions of formic acid (HC00H) were found effective as they provided the necessary acidity without contributing any undesirable anions into an already complex mixture. However, there are several drawbacks to existing hydrolysis processes.
Among the most serious of the drawbacks is foaming within the hydrolysis reactor caused by the concentrated tetraphenylborate precipitate slurry trapping evolved gases. Heating the precipitate slurry during the hydrolysis reaction causes gases to be evolved from solution and these gases are trapped in the precipitate slurry to form a stable foam. The extent of this problem is apparent from the fact that at a suitable reaction temperature, the resulting foam layer more than doubles the apparent volume of the solution. The foaming problem is magnified by the fact that trace contaminants within the precipitate slurry react in the acidic reaction solution to produce other gases, such as nitrogen oxides and carbon dioxide, which add to the foam. To contain this foam, a pressure vessel which is maintained above ambient pressure is needed as a reactor. This vessel is sealed prior to heating and may be reopened only after the hydrolysis reactions have occurred and the foam has been dispersed. For example, an earlier, uncatalyzed, hydrolysis reaction required a pressure vessel with the reaction occurring at a pressure ranging from about 25 to 50 pounds per square inch gauge for a period of about 5 hours. For a number of reasons, including safety considerations, process complexity, and limitations on service pressure, it was desirable to avoid the requirement of elevated pressure for the reaction to proceed efficiently.
In addition, and just as importantly, earlier hydrolysis reactions achieve no better than about 75 percent removal of the organic compounds. Thus, the considerations of safety, complexity, and efficiency were motivating factors to develop a hydrolysis process that would overcome the limitations and deficiencies of earlier processes.